Flooding method using salt-insensitive polymers for better mobility control

ABSTRACT

Improved flooding of an oil-bearing subterranean reservoir with an aqueous slug containing a salt-sensitive polymer (example of such a polymer is a partially hydrolyzed, high molecular weight polyacrylamide) and a subsequent drive water which, due to the salt content, tends to degrade the back portion of the aqueous polymer slug, is realized by following the aqueous, saltsensitive polymer slug with a sufficient volume of an aqueous biopolymer slug to &#39;&#39;&#39;&#39;insulate&#39;&#39;&#39;&#39; the salt-sensitive polymer from the drive water. Drive water having salt concentrations above 1,000 ppm (parts per million) of TDS (total dissolved solids) and especially those waters containing above 50 ppm of polyvalent cations, tends to adversely affect the mobility reducing capabilities of a salt-sensitive polymer. More efficient mobility control is obtained by interjecting the biopolymer slug between the salt-sensitive polymer slug and the drive water. Optionally, a miscible or miscible-like slug can precede the salt-sensitive polymer slug.

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United States Patent Knight INSENSITIVE POLYMERS FOR BETTER MOBILITYCONTROL [72] Inventor: Bruce L. Knight, Littleton, C010. [73] Assignee:Marathon Oil Company, Findlay,

Ohio

[22] Filed: June 25, 1971 [21] Appl. No.: 156,937

[52] US. Cl ..l66/246, 166/273 [51] Int. Cl. ..E21b 43/22 [58] Field ofSearch ..l66/246, 273, 274

[56] References Cited UNITED STATES PATENTS 3,053,765 9/1962 Sparks..l66/274 3,087,539 4/1963 Maurer, Jr 166/274 3,305,016 2/1967 Lindblomet al. 166/246 3,434,542 3/1969 Dotson et al. 166/273 3,437,141 4/1969Brandner et al.. 166/273 3,467,187 9/1969 Gogarty et a1 166/2733,512,586 5/1970 l-lolm.... 166/273 3,532,166 10/1970 Williams.....l66/274 3,581,824 6/1971 Hurd ..l66/246 [451 Dec. 26, 1972 PrimaryExaminer-Stephen J. Novosad Attorney-Joseph C. Herring et al.

[5 7 ABSTRACT water having salt concentrations above 1,000 ppm-- (partsper million) of TDS (total dissolved solids) and especially those waterscontaining above 50 ppm of polyvalent cations, tends to adversely affectthe mobility reducing capabilities of a salt-sensitive polymer. Moreefficient mobility control is obtained by interjecting the biopolymerslug between the salt-sensitive polymer slug and the drive water.Optionally, a miscible or miscible-like slug can precede thesalt-sensitive polymer slug.

10 Claims, No Drawings CROSS REFERENCE TO RELATED APPLICATIONS US. Pat.application titled Economical Mobility Control in Oil RecoveryProcesses, U. S. Ser. No. 76,140, filed Sept. 28, 1970, inventor BruceL. Knight, defines improved mobility control by following an aqueousslug containing a polyelectrolyte with an aqueous slug containing abiopolymer. The polyelectrolyte tends to reduce the permeability of theformation while the biopolymer increases the viscosity of the water,thus a two-flow effect is obtained.

BACKGROUND OF THE INVENTION 1. Field of the lnvention This inventionrelates to improved mobility control wherein an aqueous, salt-sensitivepolymer slug-is injected into an oil-bearing subterranean formation anddisplaced toward a production means to recover crude oil therethrough.The polymer slug can be preceded by a miscible or miscible-likedisplacing slug, e.g. a micellar dispersion or any slug which willdisplace oil. This invention teaches that an aqueous biopolymer sluginjected behind the aqueous salt-sensitive polymer slug is useful toprotect the polymer from being degraded due to a subsequently injecteddrive water containing relatively large concentrations of electrolyte,e.g. above 1,000 ppm of salts.

2. Prior Art to the Invention It is known that high molecular weightpolyelectrolytes, such as the partially hydrolyzed, high molecularweight polyacrylamides, are useful to obtain improved mobility controland increase oil recoveries in an oil-recovery process. However, thesepolyelectrolytes are generally salt-sensitive, i.e. relatively highconcentrations of salt tend to adversely influence the mobility controlimparted by these polymers to aqueous flooding systems. For example,concentrations above 1,000 ppm of salt in the water, on contact with anaqueous slug containing the polyelectrolyte, tend to reduce the desiredmobility effectiveness which the polyelectrolyte imparts to the aqueoussolution. To overcome this adversity, larger concentrations of thepolyelectrolyte are generally added to the aqueous solution.

ln some reservoirs, where the salt concentration of the drive water canreach 100,000 ppm, and even with salt concentrations above 50 ppm ofdivalent cations, the economics of a waterflooding process or anyflooding process using salt-sensitive polymers is questionable.

SUMMARY OF THE INVENTION Applicant has discovered a novel process forimproving the mobility control of an oil-recovery process wherein asalt-sensitive polymer is used to impart mobility control. This isaccomplished by injecting after the aqueous polymer slug, e.g. anaqueous solution containing a partially hydrolyzed, high molecularweight polyacrylamide, a buffer" slug or insulating slug, e.g. l-50 percent formation pore volume, containing a biopolymer. The effect of thebiopolymer is to prevent a subsequent drive water which containssufficient amounts of salt from adversely influencing the mobilitycharacteristics the polymer normally imparts to the aqueous slug. Amiscible or miscible-like slug can be injected previous to thesalt-sensitive polymer slug, e.g. in a tertiary recovery process.

PREFERRED EMBODlMENTS OF THE lNVENTlON The salt-sensitive polymers towhich this invention relates are'those used to impart desired mobilitycontrol to oil recovery processes. ln general, these salt-sensitivepolymers are known as mobility buffer agents, viscosity impartingagents, or thickening agents for aqueous solutions the processes inwhich they are used result in increases in oil recovery attributedmostly to improvement in pattern sweep and conformance or merelyimproved mobility control of the oil recovery process. Examples of suchpolymers include the partially hydrolyzed, high molecular weightpolyacrylamides marketed under the trade name Pusher polymers by DowChemical Company, Midland,-Mieh. Other examples of such polymers includepartially hydrolyzed, high molecular weight polyacrylamides marketedunder the trade names Uni-Perm and HiVis by Betz Laboratories, lnc.,Trevose, Pa., and partially hydrolyzed, high molecular weightpolyacrylamides marketed as Calgon 454 by Calgon Corp., Pittsburgh, Pa.The salt-sensitive polymers in aqueous solutions are adverselyinfluenced by high salt concentrations, i.e. the viscosity of theaqueous solution is degraded and permeability reduction capability isalso reduced. For example, a 700 ppm solution of Dow 700 Pusher polymerin water containing 500 ppm of total dissolved solids, has a Brookfieldviscosity of about 30 cp at 72F; but, the same polymer concentration in2% NaCl solution has a viscosity of 4 cp at the same temperature. Also,certain salts can precipitate the polymer out of solution, e.g. polymeris precipitated when an aqueous solution containing 700 ppm of Dow 700Pusher polymer comes in contact with water containing 200 ppm of Fe ion.

ln general, these salt-sensitive polymers are used in processesto obtainincreased oil recovery due to improvement in pattern sweep andconformance, mobility control with or without another fluid bank, etc.

The aqueous slug of injected polymer is displaced by a drive water.Where the drive water contains large concentrations of salts (e.g. above500l,000 ppm), the front portion of the drive water tends to degrade theaqueous slug containing the salt-sensitive polymer, and an inefficientprocess results.

Where the salt-sensitive polymer is used in conjunc? tion with apreviously injected miscible or miscible-like displacing slug, retentionof the mobility control imparted by the salt-sensitive polymer isimportant. U. S. Pat. Nos. 3,254,714 to Gogarty et a1; 3,497,006 toJones et a1; and 3,506,070 to Jones are examples of such a process.Examples of displacing slugs useful before the polymer slug includeanhydrous soluble oils, hydrous soluble oils, miscible slugs such asalcohol, aldehyde, LPG, or like slug, etc.

The salbsensitive polymer is generally injected in an aqueous solutionslug in concentrations of about to about 5,000 and preferably 200 toabout 2,000 and most preferably about 500 to about 1,500 ppm.

Volume amounts of the aqueous slug include about 20' to about 100 percent, preferably 30 to 70 per cent, and most preferably 35 to 55 percent formation pore volume. The particular formation, the economics ofthe overall process, i.e. the value of the crude oil, the rate of returnon investment, etc., will govern the volume amount of aqueous polymerslug injected and also the concentration of the polymer. Also, the saltor electrolyte concentration within the interstitial water will dictatethe concentration of polymer, Le, a high concentration is needed wherethe salt concentration is relatively high in the interstitial water.

Applicants invention involves injecting behind the aqueoussalt-sensitive polymer slug an aqueous biopolymer slug to substantiallyprotect thesaltsensitive polymer from the salts of electrolytes withinthe drive water. Volume amounts of such a protective slug include 1 to50 per cent, preferably 2 to 30 per cent and more preferably 5 to percent formation pore volume. The protective slug can contain about to1,000 or greater, preferably 75 to 800, and more preferably 150 to 700ppm of the biopolymer. Of course, where the drive water contains arelatively large concentration of salts or electrolytes, e.g. 100,000ppm, the concentration of the biopolymer and the volume of thebiopolymer should be increased. Examples of useful biopolymers includepolysaccharides and derivatives thereof. A specific example is Kelzan-M, a polysaccharide polymer marketed by Xanco Division of Kelco ChemicalCo., San Diego, Calif, USA. Other like polymers which are notsubstantially sensitive to the salts in the interstitial water are alsouseful within the protective slug. Otheradditives may be incorporatedinto the slug, such as corrosion inhibitors, oxygen scavengers, etc.

After the slug containing the salt-sensitive polymer and the slugcontaining the biopolymer are injected into the reservoir, drive wateris injected to displace the previous two slugs toward the productionwell to recover crude oil therethrough. As mentioned earlier, a miscibleor miscible-likeslug can be injected into the formation previous tothe'injection of the salt-sensitive polymer. The concentration of saltswithin the drive water, to be applicable with this invention, are thosecontaining above 500 and preferably above 1,000 and can contain up to100,000 ppm or more of total dis solved solids within the water.Generally speaking, the drive water always has a higher saltconcentration than the water needed to make up the salt-sensitivepolymer solution. This is understandably so, since you would not use ahigh concentration of salt within the water as a diluent with asalt-sensitive polymer. Therefore, the salt concentration within thedrive water is, without exception, greater than the salt concentrationwithin the saltsensitive polymer diluent. Also, the drive water cancontain a relatively high concentration of divalent cations, e.g.calcium and magnesium; this concentration may be greater than 50 ppm. Byusing the aqueous slug containing the biopolymer, the high concentrationof salt within the drive water is buffered from adversely influencingthe mobility characteristics imparted by the salt-sensitive polymer tothe aqueous polymer slug.

The following examples are presented to teach working embodiments of theinvention. Unless otherwise specified, all percents are based on volume.

a was:

EXAMPLE I A Berea slab core 2-feet wide, 2-feet long and 2- inches thickrepresents one-quarter of a confined fivespot pattern. The slab is firstsaturated with brine, then flooded to irreducible water saturation withcrude oil having a viscosity of about 6 cp at ambient temperature.Thereafter, the core is waterflooded to residual oil saturation with thebrine. The core is then flooded with various fluids in the followingsequence:

1. 5 per cent formation pore volume of a micellar dispersion containing9.6 per cent petroleum sulfonates (average equivalent weight about 420,about 62 per cent active), 38 per cent of crude oil (viscosity 9 cp atambient temperature), 50. per cent aqueous medium, 1.0 per cent primaryhexanol, and L4 per cent primary amyl alcohol;

2. 5 per cent formation pore volume of an aqueous solution containing2400 ppm of Dow 700 Pusher polymer;

3. 25 per cent formation pore volume of an aqueous slug containing 600ppm of Dow 700 Pusher polymer;

. 20 per cent formation pore volume of an aqueous slug containing 200ppm of Kelzan-M; and thereafter,

5. drive water containing 12,000 ppm of total dissolved solids.

The water used as the diluent in 2, 3, and 4 contains up to 500 ppm ofsalts. The drive water is injected until oil production ceases. About 72per cent of the oil is recovered from the slab.

EXAMPLE II The procedure of Example I is repeated except Dow 700 Pusherpolymer is substituted for the Kelzan-M material, ln this case, only 55per cent of the oil is recovered, even though the total amount ofpolymer injected in both cases is the same.

Example ll, as compared to Example 1, indicates that the aqueous slug ofthe biopolymer is very beneficial to improve oil recovery wherein adrive water containing high concentrations of salts is used and asalt-sensitive polymer is used.

It is not intended that this invention be limited by the specificstaught above. Rather, all equivalents obvious to those skilled in theart are intended to be incorporated within the scope of the invention asdefined within the specification and appended claims.

What is claimed is:

1. An improved process for recovering crude oil from an oil-bearingsubterranean formation wherein an aqueous slug containing asalt-sensitive polymer is injected into the formation to impart desiredmobility control to the process and the slug is displaced by a waterdrive toward a production well to recover crude oil therethrough andwherein the drive water contains a sufficient salt concentration toadversely influence the mobility characteristic imparted by thesalt-sensitive polymer, the improvement comprising injecting after theaqueous slug and before the drive water, sufficient amounts of anaqueous slug containing a biopolymer to substantially protect thesalt-sensitive polymer in the back portion of the aqueous slug frombeing degraded by the salts within the drive water.

2. The process of claim 1 wherein the salt-sensitive polymer is apartially hydrolyzed, high molecular weight polyacrylamide.

3. The process of claim 1 wherein about 1 to about 50 per cent formationpore volume of the aqueous slug containing the biopolymer is injectedinto the forma tion.

4. The process of claim 1 wherein about 2 to about 30 per cent formationpore volume of the aqueous slug containing the biopolymer is injectedinto the formation.

5. The process of claim 1 wherein the biopolymer concentration in theaqueous slug is about 25 to about 1,000 ppm.

6. The process of claim 1 wherein a micellar disper-' sion is injectedinto the formation previous to the injection of the aqueous slugcontaining the salt-sensitive polymer.

7. An improved process for recovering crude oil from an oil-bearingsubterranean formation wherein an aqueous slug containing a partiallyhydrolyzed, high molecular weight polyacrylamide is injected into theformation to impart desired mobility control to the process, and whereinthe aqueous slug is displaced by a drive water toward a production meansto recover crude oil therethrough and wherein the water drive contains asalt concentration above about 500l,000 ppm which adversely influencesthe mobility characteristics imparted by the polyacrylamide, theimprovement comprising injecting after the aqueous polyacrylamide slugabout 1 to about 50 per cent formation pore volume of an aqueous slugcontaining a biopolymer.

8. The process of claim 7 wherein the biopolymer concentration is about25 to about 1,000 ppm within the aqueous slug containing the biopolymer.

9. The process of claim 7 wherein a micellar dispersion is injectedprevious to the injection of the aqueous slug containing thepolyacrylamide.

10. The process of claim 7 wherein about 2 to about 30 per centformation pore volume of the aqueous slug containing the biopolymer isinjected.

2. The process of claim 1 wherein the salt-sensitive polymer is apartially hydrolyzed, high molecular weight polyacrylamide.
 3. Theprocess of claim 1 wherein about 1 to about 50 per cent formation porevolume of the aqueous slug containing the biopolymer is injected intothe formation.
 4. The process of claim 1 wherein about 2 to about 30 percent formation pore volume of the aqueous slug containing the biopolymeris injected into the formation.
 5. The process of claim 1 wherein thebiopolymer concentration in the aqueous slug is about 25 to about 1,000ppm.
 6. The process of claim 1 wherein a micellar dispersion is injectedinto the formation previous to the injection of the aqueous slugcontaining the salt-sensitive polymer.
 7. An improved process forrecovering crude oil from an oil-bearing subterranean formation whereinan aqueous slug containing a partially hydrolyzed, high molecular weightpolyacrylamide is injected into the formation to impart desired mobilitycontrol to the process, and wherein the aqueous slug is displaced by adrive water toward a production means to recover crude oil therethroughand wherein the water drive contains a salt concentration above about500- 1,000 ppm which adversely influences the mobility characteristicsimparted by the polyacrylamide, the improvement comprising injectingafter the aqueous polyacrylamide slug about 1 to about 50 per centformation pore volume of an aqueous slug containing a biopolymer.
 8. Theprocess of claim 7 wherein the biopolymer concentration is about 25 toabout 1,000 ppm within the aqueous slug containing the biopolymer. 9.The process of claim 7 wherein a micellar dispersion is injectedprevious to the injection of the aqueous slug containing thepolyacrylamide.
 10. The process of claim 7 wherein about 2 to about 30per cent formation pore volume of the aqueous slug containing thebiopolymer is injected.